The objective of this proposal is to study the effects of laser recanalization in arteries occluded by atherosclerosis and thrombosis. The benefits as well as the side effects will be evaluated in the immediate and long-term periods after laser recanalization in various animal models and in man. Also evaluation of the laser effects on myocardial muscle will be done for purposes of His Bundle and arrhythmia foci ablation. The laser has been shown to recanalize vascular obstructions and to create complete heart block by interruption of His Bundle in dogs. However, despite the preliminary results, several questions remain unanswered: 1) What is the ideal laser wavelength for recanalization and myocardial muscle vaporization; 2) What effect does temperature have on the artery and surrounding tissues; 3) Is there production of tissue debris; 4) Will characterization of the optical properties improve laser applications; 5) Will chromophores enhance laser energy localization and increase its safety; 6) What is the thrombogenic potential of laser applications; 7) Can optical fibers be designed to reduce perforations, 8) Can adequate channels be made in humans; if so, will these remain patent and for how long; 9) Can the laser be used as a biological probe to identify tissue during recanalization. Three areas of investigation will attempt to address these questions: 1) In vitro studies will investigate basic laser-tissue interactions. These include histologic, biochemical and optical studies of normal and atherosclerotic arteries, and cardiac muscle for selection of the optimal wavelength and parameters for vaporization. Chromophores will be used to enhance this process. These data will allow for the optical characterization of these tissues, thus making the laser a biological probe. Thermography and thermocouples will be used to measure temperature changes during arterial recanalization to assess thermal injury to the surrounding tissue. Debris will be collected and measured. 2) In vivo animal studies will include testing of various optical fiber devices to evaluate recanalization and His Bundle ablation techniques. 3) A clinical trial will provide data on the adequacy and longevity of vascular channels made in patients with occluded peripheral arteries. This study will utilize angioscopic, angiographic and physiologic evaluation to assess the maintenance of blood flow to areas of occlusive vascular disease.